During the ‘Retake Normandy’ campaign, my screen suddenly turned into a faded, over-saturated window.
Nothing on the screen was distinguishable:
Specifically, this happened as I was sending troops (cavalry, siege. infantry) to siege the outer wall of the North-Eastern stone walls that protect the main objective. As seen here:
The troops entered the space and the system saved the game. Then it happened.
I also had a battle ensuing elsewhere:
Perhaps a lot going on at once?